The present invention relates to a method for writing servo signals for positioning a read-write head into a magnetic disk surface by means of magnetic transfer technique in a hard disk drive (HDD) which uses magnetic film as recording material, is now a main stream of external memory. In particular, the present invention relates to magnetic transfer technique in a perpendicular magnetic recording medium, in which the direction of recorded magnetization is perpendicular to the medium surface.
The recording density of a magnetic recording medium for HDD is as high as 20 Gbit/in2 in the development stage at present and the memory capacity is increasing in the rate of 60% a year. With the enhancement of recording density, the size per bit of the magnetic material is diminishing. It is becoming difficult to read-write data by so-called longitudinal magnetic recording system, in which the direction of recorded magnetization is a longitudinal direction, that is, a direction in the surface of the magnetic recording medium. Accordingly, a so-called perpendicular magnetic recording system has been proposed, in which the direction of recorded magnetization is perpendicular to the surface of the magnetic recording medium. A perpendicular magnetic recording medium includes a magnetic recording layer of hard magnetic substance and a backing layer of soft magnetic material 16 that serves to concentrate the magnetic flux generated by the magnetic head and used for recording to the magnetic recording layer.
In a common HDD, recording and reproduction of data are performed with a magnetic head floating over the surface of the rotating magnetic recording medium of a hollow disk shape. The floating is effected by a floating mechanism called a slider and the flying height is several tens of nm. Bit information on the magnetic recording medium is stored in the data tracks concentrically positioned on the medium. In the recording and reproduction of data, a read-write head moves to the target data track with high speed. The recording surface of the magnetic recording medium contains preformat information that includes tracking servo signals for detecting relative position between the head and the data track, and address signals or regenerative clock signals. The preformat information corresponding to a data track is recorded on a circle concentric with the data track at certain angular intervals. The preformat information of the whole magnetic recording medium is recorded on linear preformat regions 35 with a sector form, which is substantially a linear form, arranged at certain angular intervals as shown in FIG. 11. For preventing the center of the row of preformat signals from diverting from the center of the magnetic recording medium or from the center of the orbit of the read-write head, the preformat information has been conventionally recorded using a special writing apparatus called a track writer after mounting the magnetic recording medium on the HDD.
Accompanying the above-described enhancement of recording density, the recording density of the preformat information is also increasing, to prolong the time for writing the preformat information. This is becoming a serious factor to lower efficiency in HDD production and to raise HDD cost.
Recently, a method for writing the preformat information to a magnetic recording medium has been proposed, in which the information is written to the recording medium in an a real manner by means of magnetic transfer technique using a master disk carrying the preformat information, in place of writing to each of the tracks with linear manner using the signal-writing head of a servo slider. For example, Japanese Unexamined Patent Application Publication (KOKAI) No. H10-40544 discloses a method for transferring preformat information to a longitudinal magnetic recording medium using a master disk that has protrusions and recesses, the protrusions being composed of ferromagnetic material. Japanese Unexamined Patent Application Publication (KOKAI) No. H11-25455 discloses a method for close contact between a master disk and a magnetic recording medium in the magnetic transfer process by means of supplying and exhausting air in the groove of the master disk. However, these references don""t disclose a magnetic transfer method for a perpendicular magnetic recording medium.
A technology for magnetic transfer to a perpendicular magnetic recording medium that is a recording medium where the direction of magnetization for recording is perpendicular to the medium surface has never been devised. FIG. 3(A) and FIG. 3(B) show an initialization step and a transfer step, respectively, in a devised magnetic transfer method to a perpendicular recording medium 1. The structure of a master disk 2 used in the method is the same as that in the case of a longitudinal magnetic recording medium. In the step for initializing the perpendicular magnetic recording medium 1, a magnetic field perpendicular to the magnetic recording medium 1 surface is applied using single magnetic pole heads 16 as illustrated in the FIG. 3(A). Two single magnetic pole heads 16 and 16 are symmetrically arranged such that the two magnetic poles of different polarity are opposing with the medium put between the poles. This arrangement is taken because (1) spread of magnetic field in the longitudinal direction is smaller than in the case using one single magnetic pole head 16, and (2) only perpendicular component of magnetic field is applied to the medium. In the step for transferring, the master disk 2 is closely contacted to the medium, and two single magnetic pole heads 16 and 16 are likewise symmetrically arranged such that the two magnetic poles of different polarity are opposing with the master disk 2 and the medium put between the poles, as shown in FIG. 3(B). Here, the direction of the magnetic field is reversed in the transfer step from in the initialization step.
FIG. 4 also shows a prior art, in which one single magnetic pole head 16 having a yoke 7 is used. In the opposite side of the medium to the head, a thick plate made of soft magnetic substance called a back pole 8 is provided in place of a single magnetic pole head. This arrangement leads the magnetic flux 9 generated by the permanent magnet 4 through the yoke 7 and the back pole 8 so that the magnetic field is perpendicular to the medium surface 1.
FIG. 5(A), FIG. 5(B) and FIG. 5(C) show a principle of magnetic transfer in a perpendicular magnetic recording medium 1. FIG. 5(B) and FIG. 5(C) show magnetic field distribution when a magnetic field is applied to the master disk 2 and the recording medium 1 closely contacted each other, with the field direction perpendicular to their surfaces, using lines of magnetic force 13 and a graph showing the distribution of perpendicular component of the magnetic field. When a uniform and perpendicular magnetic field is applied to the master disk 2, the uniform flux is concentrated to the embedded soft-magnetic material 6 having high permeability. Hence, magnetic flux density or magnetic field intensity in the recording medium is large in the position the soft-magnetic material 6 is arranged and small in the position in the space between the soft-magnetic material 6, as shown in FIG. 5(B). If a perpendicular magnetic recording medium 1 having coercive force Hc of proper value is used, the direction of magnetization in the region of the medium beneath the soft-magnetic material 6 can be reversed in the transfer step from the direction in the initialization step; in FIG. 5(A), the direction is reversed from upward to downward. In the region of the medium beneath the space area where the soft-magnetic material 6 does not exist, the magnetization is not reversed and the direction of magnetization holds. FIG. 5(A) illustrates above-described situation and the distribution of intensity of signals when the magnetization transferred to the perpendicular magnetic recording medium 1 is read out.
According to the principle of magnetic transfer in a perpendicular magnetic recording medium 1 shown in FIG. 5(A), FIG. 5(B) and FIG. 5(C), if the difference in values of magnetic field intensity at the soft-magnetic material 6 and magnetic field intensity at the space without the soft-magnetic material 6 is significant, in other words, if almost entire magnetic flux is concentrated in the soft-magnetic material 6 so that the magnetic field intensity is large at the soft-magnetic material 6 and nearly zero at the space without the soft-magnetic material 6, the magnetization in the recording layer beneath the soft-magnetic material 6 is reversed and the initial magnetization in the recording layer beneath the space without the soft-magnetic material is conserved, which is an ideal circumstance for the magnetic transfer.
Concentrating large portion of magnetic flux in the soft-magnetic material 6 requires a thicker soft-magnetic embedded layer, as well as a higher permeability of the soft magnetic substance. Thickness of the soft-magnetic material 6 is generally from 300 nm to 500 xcexcm at present. The line width in the longitudinal direction of the soft-magnetic material 6 is 1 xcexcm, or the width of the space without the soft-magnetic material 6 is 1 xcexcm. To raise the density of the servo information that is written by magnetic transfer, the length of the soft-magnetic material 6 along the circumferential direction of the magnetic recording medium must become finer. However, to obtain finer pattern, the thickness of the soft-magnetic material 6 is necessary to be thinner. Therefore, coexistence of the fine pattern and the ideal magnetic transfer is difficult.
It is therefore an object of the present invention to provide a method of magnetic transfer for a perpendicular magnetic recording medium 1 without increasing the thickness of the soft-magnetic material 6 than the present value.
The first embodiment of the present invention is a method for magnetically transferring preformat information to a magnetic recording layer of the magnetic recording medium by applying an external magnetic field in an arrangement where a master disk is in close contact with or in proximity to the magnetic recording medium, the master disk having a non-magnetic substrate and a multiple of isolated soft magnetic material on a surface or in a surface portion of the non-magnetic substrate, the soft magnetic material having a pattern corresponding to the preformat information and being disposed in a linear region. The method includes: (1) an initialization step, in which a magnetic field is applied to the magnetic recording medium in a direction perpendicular to a surface of the magnetic recording medium so that direction of magnetization of whole surface of the magnetic recording medium is aligned to one direction perpendicular to the magnetic recording medium, and (2) a magnetic transfer step, in which a magnetic field in a longitudinal direction is applied to both of the master disk and the magnetic recording medium in an arrangement where the master disk is in close contact with or in proximity to one surface or both surfaces of the magnetic recording medium.
Preferably, in the initialization step, a magnetic field is applied to the magnetic recording medium in perpendicular direction (with respect to the medium surface) of the medium using a single magnetic pole head, and in the magnetic transfer step, a magnetic field in a longitudinal direction is applied to the magnetic recording medium and the master disk using one or more ring heads each having a gap.
The second embodiment of the present invention is a method for magnetically transferring preformat information to a magnetic recording layer of the magnetic recording medium by applying an external magnetic field in an arrangement where a master disk is in close contact with or in proximity to the magnetic recording medium, the master disk having a non-magnetic substrate and a multiple of isolated soft magnetic material on a surface or in a surface portion of the non-magnetic substrate, the soft magnetic material having a pattern corresponding to the preformat information and being disposed in a linear region. The method includes: (1) a step for preparing as the magnetic recording medium an uninitialized magnetic recording medium as obtained from a manufacturing step, and (2) a magnetic transfer step, in which a magnetic field in a longitudinal direction is applied to both of the master disk and the magnetic recording medium in an arrangement where the master disk is in close contact with or in proximity to one surface or both surfaces of the magnetic recording medium.
Preferably, in the magnetic transfer step, a magnetic field in a longitudinal direction is applied to the magnetic recording medium and the master disk using one or more ring heads each having a gap.
In a magnetic transfer step of first and second embodiments of the present invention, as heads for transfer magnet, preferably, two ring heads each having a gap are opposed each other, and the magnetic recording medium and the master disk are positioned in a center of the gaps of the two ring heads.
In a magnetic transfer step of first and second embodiments of the method of magnetic transfer for a perpendicular magnetic recording medium of the present invention, preferably, the magnetic recording medium has an axis of easy magnetization in a direction perpendicular to a surface of the medium.
In a magnetic transfer step of first and second embodiments of the method of magnetic transfer for a perpendicular magnetic recording medium of the present invention, preferably, the direction of recorded magnetization in said magnetic recording medium is perpendicular to said magnetic recording medium.